A grinding process can cause high levels of stress on delicate materials as the grinding medium tears at the surface of the material, often generating a lot of heat and potentially leaving an uneven surface behind. The damage caused during grinding will also propogate through the material, often producing considerable sub-surface damage. Where a grinding pad is used on a revolving platen, it will always wear unevenly due to the difference in relative movement speed and pad area across the pad diameter, this means that the grinding effect on the sample will change during the process, reducing the accuracy of the sample and the uniformity.
Lapping is a more gentle process as the abrasive is held in a carrier fluid which passes as a liquid between the sample and the lapping plate, and as the liquid flows it performs in a more uniform manner, resulting in more uniform samples. The damage to the material during the lapping process is much lessened and more predictable, and the chance of sample breakage is much reduced.